1. Field of the Invention
The present invention relates generally to systems and methods for imaging tissue using magnetic resonance imaging (MRI) techniques, and more particularly to obtaining high accuracy temperature measurements of a tissue mass being heated by compensating for errors caused by motion of the tissue mass and/or dynamic changes in magnetic field intensity or uniformity during the acquisition of MR images used for temperature measurement.
2. Background
Certain types of tissues, such as cancerous tumors, can be destroyed by heat. One conventional way to heat these tissues is by directing laser energy into the tissue using, e.g., a laser source carried by a catheter. Another conventional way is to focus high intensity, ultrasonic acoustic waves into the tissue using, e.g., a phased-array of piezoelectric transducers. Both of these approaches can reduce or even eliminate the need for invasive surgery to remove the tissue.
Of critical importance to the process is verifying that a sufficient temperature was reached during each application of ultrasonic energy to kill the target tissue structure, or portion thereof being heated. This can be done by measuring the temperature change (rise) of the portion of the tissue structure being heated during the heating process using MR imaging techniques.
One known method of measuring temperature change using MR techniques exploits the temperature dependence of the proton resonant frequency (PRF) in water. The temperature dependence of the PRF is primarily due to temperature-induced induced rupture, stretching, or bending of the hydrogen bonds in water. The temperature dependence of pure water is 0.0107 ppm/xc2x0 C., and the temperature dependence of other water-based tissues is close to this value.
Because of a non-homogenous magnetic field within the MRI machine, absolute PRF measurements are not possible. Instead, changes in PRF are measured by first taking a MR image before the delivery of heat, and subtracting this baseline value from subsequent measurements. Notably, the total imaging time must be kept relatively short for the baseline value to remain relatively stable, since drifts in the magnetic field can occur over time.
The temperature-induced changes in PRF are then estimated by measuring changes in phase of the MR signal, or frequency shift in certain MR imaging sequences. Unfortunately, movements of the object being imaged or dynamic changes in magnetic field intensity and/or uniformity during the MR imaging process (i.e., between successively acquired images) can also induce phase shifts, which can be misinterpreted as a temperature-induced phase shift. As a result, a given phase shift might be attributable to any one of temperature change, motion, changes in magnetic field, or some combination thereof, between acquired images. This ambiguity makes it difficult to determine the tissue mass temperature change relying only on the MR signals from the tissue mass being heated. Such motion and non-uniform field xe2x80x9cdistortionxe2x80x9d may have differing sources acting alone or in combination, such as patient motion, breathing or other dynamic organ movement (e.g., the heart), or blood moving through a blood vessel located in or adjacent to the target tissue region.
In accordance with a general aspect of the invention, MR temperature change monitoring of a heated portion of a tissue mass undergoing thermal treatment is adjusted to compensate apparent temperature change measurements caused by movement of the mass and/or changes in magnetic field between acquisition of MR images by subtracting out similar apparent temperature change measurements made of one or more unheated portions of the tissue mass located in a neighborhood of the heated portion, which form a temperature bias map of the tissue mass region.
One implementation of the invention is a system and method for determining a change in temperature of a heated portion of a mass using magnetic resonance imaging techniques.
In one embodiment, the system and method include measuring an apparent change in temperature of the heated portion of the mass; measuring an apparent change in temperature of an unheated portion of the mass located in a neighborhood of the heated portion; and adjusting the measured apparent change in temperature of the heated portion based at least in part on the measured apparent change in temperature of the unheated portion.
In one embodiment, the apparent change in temperatures of the respective heated and unheated portions of the mass are determined by measuring a characteristic related to temperature both before and after the heated portion is heated, and then deriving a temperature change based on a change in the measured characteristic. By way of non-limiting example, the measured characteristic may be the phase of an electromagnetic signal emitted from the respective portion, wherein the apparent temperature change is derived from a phase shift in the signal.
In order to increase accuracy of the temperature change measurement, the apparent change in temperatures of multiple unheated portions of the mass in the neighborhood of the heated portion may be used to more precisely identify the temperature bias field in the area of the heated portion. In order to reduce effects in the apparent temperature changes of unheated portions directly adjacent to the heated portion caused by some thermal energy transfer, the relative locations of the unheated portions with respect to the heated portion may also be taken into account.
Other aspects and features of the invention will become apparent in view of the disclosed and described preferred embodiments.